BP's Macondo well spilled only 3.26 million barrels of oil during the 87 days that followed the April 20, 2010 blowout that sank the Deepwater Horizon drilling rig, killing 11 workers, a witness for the company testified Thursday.

Witness: Only 3.26m barrels from BP well

Martin Blunt, an assistant professor of petroleum engineering at Great Britain’s Imperial College, said his estimate takes into account the geology of the area of the Gulf of Mexico where the Macondo reservoir was located, unlike the 5 million or more barrels estimated by expert witnesses for the Justice Department.

Blunt’s testimony comes in the second part of phase 2 of the civil damages trial of BP and its drilling partners, which is aimed at determining how much oil entered the Gulf of Mexico during the spill.

U.S. District Judge Carl Barbier, who is hearing the case without a jury, is expected to weigh the competing testimony to determine how many barrels were spilled for purposes of deciding how much BP and its drilling partners should pay in Clean Water Act fines.

The federal law allows him to charge the responsible parties up to $1,100 per barrel if he finds they acted with negligence, and up to $4,300 per barrel, if they acted with gross negligence or willful neglect.

Using Blunt’s estimate, BP contends it is liable for only 2.45 million barrels of oil, since Barbier already has ruled that the fines will not cover 810,000 barrels of oil that were collected directly from the well at the surface and taken to refineries, with the revenue donated to charities.